The distance between the cities is $\displaystyle\frac{{{900},{000}\ \text{ meters}}}{{1}}\cdot\frac{{{1}\ \text{ kilometer}}}{{{1},{000}\ \text{ meters}}}\usepackage{cmbright}\Huge\$ , or 900 kilometers.

$\displaystyle\frac{{{900}\ \text{ km}}}{{n}}=\frac{{{180}\ \text{ km}}}{{{1}\ \text{ hour}}}\usepackage{cmbright}\Huge\$

$\displaystyle{n}=\frac{{900}}{{180}}={5}\usepackage{cmbright}\Huge\$